1. Field of the Invention
The present invention relates to a signal processing apparatus for an optical or magnetic disc memory device which utilizes light or magnetism in order to record or reproduce signals, especially to a signal processing apparatus, which renders a high density recording on a disc memory possible and is suited for application to a random access memory.
2. Description of the Related Art
Disc type memory devices, such as optical disc memories, magnetic disc memories and magneto-optic disc memories, usually have a large storage capacity. As is well known, therefore, they are widely employed not only for audio or video signal recording media in consumer appliances, but also for external data storages in electronic data processing systems.
Conventionally, there are two methods of driving a memory disc; one is a constant angular velocity method (called a CAV, hereinafter) and the other a constant linear velocity method (called a CLV, hereinafter). In the CAV, a memory disc is rotated at a constant angular velocity irrespective of the position of a pickup relative to the disc, i.e., whether the pickup is facing the inner-circumferential portion or the outer-circumferential portion of the disc, whereas, in the CLV, the angular velocity of a rotating memory disc is varied in accordance with the position of a pickup so that the linear velocity of a portion of the disc facing the pickup becomes constant.
In a disc memory, digital signals are bit by bit recorded at pits provided on a disc, and such pits are arranged along plural tracks which are defined on the disc in the circumferential direction thereof. Further, in the CAV, the pits must be so arranged that the interval of two adjacent pits on a track becomes large in the outer tracks, so that a pickup can always pass between two adjacent pits in the constant time and hence the frequency of the signal read out by the pickup becomes constant irrespective of the position of the pickup.
In the CLV, however, the interval of adjacent two pits are maintained constant over the whole tracks on a memory disc. The angular velocity of the rotation of the disc is so changed that it becomes large, when a pickup faces the inner tracks of the disc and small, when the pickup faces the outer tracks, whereby the pickup can always pass between two adjacent pits in the constant time, i.e., the linear velocity of a portion of the disc facing the pickup always becomes constant. As a result, in the same manner as in the CAV, the frequency of the signal read out by the pickup becomes constant irrespective of the change of the angular velocity of the disc.
Compared with the CAV, the CLV can realize a disc memory device with a large storage capacity. This is because, in the CAV, the intervals of two adjacent pits on outer tracks are inevitable to become larger than those on inner tracks, so that the bit density decreases in the outer tracks. On the other hand, the intervals of two adjacent pits in the CLV are always constant in the whole tracks, which can be selected at the minimal value allowed by the resolution of the pickup. Therefore, the bit density on the outer tracks can be heightened in the same manner as on the inner tracks.
By the way, when a disc memory is applied to a so called random access memory, a pickup is required to be able to quickly move in the radial direction of the disc. As already described, in the CLV, the angular velocity of the rotation of a disc must be changed in accordance with the position of a pickup. If, therefore, the CLV is applied to the random access memory, the angular velocity of the disc must be changed in response to the quick movement of the pickup. Practically, it is not possible to quickly change the number of revolutions of the disc, which rotates at the high angular velocity. Therefore, the CLV is not suited for the random access memory.
As mentioned above, there has been no disc memory which has the large storage capacity and is suited for a random access memory. If a disc is provided with equal-interval pits along tracks and rotated at the constant angular velocity, a disc memory device having the large storage capacity and suited for the random access memory can be realized. With this, however, the time necessary for passage of two adjacent pits under the pickup is varied in accordance with the position of the pickup. As a result, there occurs the problem, as understood from the foregoing description, that the frequency of the signal read out by the pickup changes in accordance with the position of the pickup relative to the disc.